Distinct cellular pools of perilipin 5 point to roles in lipid trafficking

Plin5: A potential therapeutic target for type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a kind of metabolic disease characterized by aberrant insulin secretion as a result of -cell loss or injury, or by impaired insulin sensitivity of peripheral tissues, which finally results in insulin resistance and a disturbance of glucose and lipid metabolism. Among them, lipid metabolism disorders lead to lipotoxicity through oxidative stress and inflammatory response, destroying the structure and function of tissues and cells. Abnormal lipid metabolism can lead to abnormal insulin signaling and disrupt glucose metabolism through a variety of pathways. Therefore, emphasizing lipid metabolism may be a crucial step in the prevention and treatment of T2DM. Plin5 is a lipid droplet surface protein, which can bi-directionally regulate lipid metabolism and plays an important role in lipolysis and fat synthesis. Plin5 can simultaneously decrease the buildup of free fatty acids in the cytoplasm, improve mitochondrial uptake of free fatty acids, speed up fatty acid oxidation through lipid drops-mitochondria interaction, and lessen lipotoxicity. In oxidative tissues like the heart, liver, and skeletal muscle, Plin5 is extensively expressed. And Plin5 is widely involved in β-cell apoptosis, insulin resistance, oxidative stress, inflammatory response and other pathological processes, which has important implications for exploring the pathogenesis of T2DM. In addition, recent studies have found that Plin5 is also closely related to T2DM and cancer, which provides a new perspective for exploring the relationship between T2DM and cancer.

The role of Perilipin 5 in pathological myocardial remodeling

Pathological cardiac remodeling (REM), caused by various pathological factors and characterized by changes in cardiac structure and geometry, is strongly associated with heart failure (HF). It damages cardiac tissue, alters energy metabolism, increases oxidative stress, and cause matrix metalloproteinase activation, cardiomyocyte hypertrophy, and interstitial fibrosis, leading to HF. REM determines the outcome of cardiovascular disease. Current treatments have limitations. REM is associated with cardiac energetic remodeling, and modulation of metabolic substrates may slow down the disease. Perilipin 5 (Plin5), positioned as a structural protein located on the surface of lipid droplets (LDs), is abundant in tissues and cells that rely on mitochondrial β-oxidation for energy production. It is the most recently identified member of the perilipin protein (PAT) family, with a notable enrichment in the cardiac muscle. Emerging evidence highlights the critical role of intracellular LD in the regulation of energy metabolism, with metabolic disruptions of LD being directly correlated with the incidence of metabolic disease. As a key barrier to LD, Plin5 is instrumental in controlling the catabolism of LD and regulating the metabolism and transport of fatty acids (FAs). As a protectant against excessive β-oxidation of free fatty acids (FFAs), Plin5 acts to isolate and neutralize overly oxidized fatty acids, thereby shielding the heart from myocardial remodeling instigated by a variety of etiological factors. This protective mechanism helps to ameliorate the progression of persistent and detrimental myocardial remodeling, which can otherwise lead to the development of severe heart failure. This systematic review attempts to delineate the metabolic disorders associated with pathological cardiac remodeling, focusing on the properties and regulatory mechanisms of Plin5. By synthesising current literature, it investigates the pivotal role of Plin5 in modulating the distinctive attributes, initiating factors, and molecular signaling networks underpinning pathological cardiac remodeling.

Omics Approaches to Study Perilipins and Their Significant Biological Role in Cardiometabolic Disorders

Lipid droplets (LDs), highly dynamic cellular organelles specialized in lipid storage and maintenance of lipid homeostasis, contain several proteins on their surface, among which the perilipin (Plin) family stands out as the most abundant group of LD-binding proteins. They play a pivotal role in influencing the behavior and functionality of LDs, regulating lipase activity, and preserving a balance between lipid synthesis and degradation, which is crucial in the development of obesity and abnormal accumulation of fat in non-adipose tissues, causing negative adverse biological effects, such as insulin resistance, mitochondrial dysfunction, and inflammation. The expression levels of Plins are often associated with various diseases, such as hepatic steatosis and atherosclerotic plaque formation. Thus, it becomes of interest to investigate the Plin roles by using appropriate "omics" approaches that may provide additional insight into the mechanisms through which these proteins contribute to cellular and tissue homeostasis. This review is intended to give an overview of the most significant omics studies focused on the characterization of Plin proteins and the identification of their potential targets involved in the development and progression of cardiovascular and cardiometabolic complications, as well as their interactors that could be useful for more efficient therapeutic and preventive approaches for patients.

Lipotoxicity of palmitic acid is associated with DGAT1 downregulation and abolished by PPARα activation in liver cells

Lipotoxicity refers to the harmful effects of excess fatty acids on metabolic health, and it can vary depending on the type of fatty acids involved. Saturated and unsaturated fatty acids exhibit distinct effects, though the precise mechanisms behind these differences remain unclear. Here, we investigated the lipotoxicity of palmitic acid (PA), a saturated fatty acid, compared with oleic acid (OA), a monounsaturated fatty acid, in the hepatic cell line HuH7. Our results demonstrated that PA, unlike OA, induces lipotoxicity, endoplasmic reticulum (ER) stress, and autophagy inhibition. Compared with OA, PA treatment leads to less lipid droplet (LD) accumulation and a significant reduction in the mRNA and protein level of diacylglycerol acyltransferase 1 (DGAT1), a key enzyme of triacylglycerol synthesis. Using modulators of ER stress and autophagy, we established that DGAT1 downregulation by PA is closely linked to these cellular pathways. Notably, the ER stress inhibitor 4-phenylbutyrate can suppress PA-induced DGAT1 downregulation. Furthermore, knockdown of DGAT1 by siRNA or with A922500, a specific DGAT1 inhibitor, resulted in cell death, even with OA. Both PA and OA increased the oxygen consumption rate; however, the increase associated with PA was only partially coupled to ATP synthesis. Importantly, treatment with GW7647 a specific PPARα agonist mitigated the lipotoxic effects of PA, restoring PA-induced ER stress, autophagy block, and DGAT1 suppression. In conclusion, our study highlights the crucial role of DGAT1 in PA-induced lipotoxicity, broadening the knowledge of the mechanisms underlying hepatic lipotoxicity and providing the basis for potential therapeutic interventions.

Perilipin1 inhibits Nosema bombycis proliferation by promoting Domeless- and Hop-mediated JAK-STAT pathway activation in Bombyx mori

Lipid droplets (LDs) are dynamic organelles that participate in the regulation of lipid metabolism and cellular homeostasis inside of cells. LD-associated proteins, also known as perilipins (PLINs), are a family of proteins found on the surface of LDs that regulate lipid metabolism, immunity, and other functions. In silkworms, pébrine disease caused by infection by the microsporidian Nosema bombycis (Nb) is a severe threat to the sericultural industry. Although we found that Nb relies on lipids from silkworms to facilitate its proliferation, the relationship between PLINs and Nb proliferation remains unknown. Here, we found Nb infection caused the accumulation of LDs in the fat bodies of silkworm larvae. The characterized perilipin1 gene (plin1) promotes the accumulation of intracellular LDs and is involved in Nb proliferation. plin1 is similar to perilipin1 in humans and is conserved in all insects. The expression of plin1 was mostly enriched in the fat body rather than in other tissues. Knockdown of plin1 enhanced Nb proliferation, whereas overexpression of plin1 inhibited its proliferation. Furthermore, we confirmed that plin1 increased the expression of the Domeless and Hop in the JAK-STAT immune pathway and inhibited Nb proliferation. Taken together, our current findings demonstrate that plin1 inhibits Nb proliferation by promoting the JAK-STAT pathway through increased expression of Domeless and Hop. This study provides new insights into the complicated connections among microsporidia pathogens, LD surface proteins, and insect immunity.IMPORTANCELipid droplets (LDs) are lipid storage sites in cells and are present in almost all animals. Many studies have found that LDs may play a role in host resistance to pathogens and are closely related to innate immunity. The present study found that a surface protein of insect lipid droplets could not only regulate the morphological changes of lipid droplets but also inhibit the proliferation of a microsporidian pathogen Nosema bombycis (Nb) by activating the JAK-STAT signaling pathway. This is the first discovery of the relationship between microsporidian pathogen and insect lipid surface protein perilipin and insect immunity.

The sterol C-24 methyltransferase encoding gene, erg6, is essential for viability of Aspergillus species

Triazoles, the most widely used class of antifungal drugs, inhibit the biosynthesis of ergosterol, a crucial component of the fungal plasma membrane. Inhibition of a separate ergosterol biosynthetic step, catalyzed by the sterol C-24 methyltransferase Erg6, reduces the virulence of pathogenic yeasts, but its effects on filamentous fungal pathogens like Aspergillus fumigatus remain unexplored. Here, we show that the lipid droplet-associated enzyme Erg6 is essential for the viability of A. fumigatus and other Aspergillus species, including A. lentulus, A. terreus, and A. nidulans. Downregulation of erg6 causes loss of sterol-rich membrane domains required for apical extension of hyphae, as well as altered sterol profiles consistent with the Erg6 enzyme functioning upstream of the triazole drug target, Cyp51A/Cyp51B. Unexpectedly, erg6-repressed strains display wild-type susceptibility against the ergosterol-active triazole and polyene antifungals. Finally, we show that erg6 repression results in significant reduction in mortality in a murine model of invasive aspergillosis. Taken together with recent studies, our work supports Erg6 as a potentially pan-fungal drug target.

Concept of lipid droplet biogenesis

Lipid droplets (LD) are functionally conserved fat storage organelles found in all cell types. LDs have a unique structure comprising of a hydrophobic core of neutral lipids (fat), triacylglycerol (TAG) and cholesterol esters (CE) surrounded by a phospholipid monolayer. LD surface is decorated by a multitude of proteins and enzymes rendering this compartment functional. Accumulating evidence suggests that LDs originate from discrete ER-subdomains, demarcated by the lipodystrophy protein seipin, however, the mechanisms of which are not well understood. LD biogenesis factors together with biophysical properties of the ER membrane orchestrate spatiotemporal regulation of LD nucleation and growth at specific ER subdomains in response to metabolic cues. Defects in LD formation manifests in several human pathologies, including obesity, lipodystrophy, ectopic fat accumulation, and insulin resistance. Here, we review recent advances in understanding the molecular events during initial stages of eukaryotic LD assembly and discuss the critical role of factors that ensure fidelity of this process.

The sterol C-24 methyltransferase encoding gene, erg6, is essential for viability of Aspergillus species

Ergosterol is a critical component of fungal plasma membranes. Although many currently available antifungal compounds target the ergosterol biosynthesis pathway for antifungal effect, current knowledge regarding ergosterol synthesis remains incomplete for filamentous fungal pathogens like Aspergillus fumigatus. Here, we show for the first time that the lipid droplet-associated sterol C-24 methyltransferase, Erg6, is essential for A. fumigatus viability. We further show that this essentiality extends to additional Aspergillus species, including A. lentulus, A. terreus, and A. nidulans. Neither the overexpression of a putative erg6 paralog, smt1, nor the exogenous addition of ergosterol could rescue erg6 deficiency. Importantly, Erg6 downregulation results in a dramatic decrease in ergosterol and accumulation in lanosterol and is further characterized by diminished sterol-rich plasma membrane domains (SRDs) at hyphal tips. Unexpectedly, erg6 repressed strains demonstrate wild-type susceptibility against the ergosterol-active triazole and polyene antifungals. Finally, repressing erg6 expression reduced fungal burden accumulation in a murine model of invasive aspergillosis. Taken together, our studies suggest that Erg6, which shows little homology to mammalian proteins, is potentially an attractive antifungal drug target for therapy of Aspergillus infections.

Plin5, a New Target in Diabetic Cardiomyopathy

Abnormal lipid accumulation is commonly observed in diabetic cardiomyopathy (DC), which can create a lipotoxic microenvironment and damage cardiomyocytes. Lipid toxicity is an important pathogenic factor due to abnormal lipid accumulation in DC. As a lipid droplet (LD) decomposition barrier, Plin5 can protect LDs from lipase decomposition and regulate lipid metabolism, which is involved in the occurrence and development of cardiovascular diseases. In recent years, studies have shown that Plin5 expression is involved in the pathogenesis of DC lipid toxicity, such as oxidative stress, mitochondrial dysfunction, endoplasmic reticulum (ER) stress, and insulin resistance (IR) and has become a key target of DC research. Therefore, understanding the relationship between Plin5 and DC progression as well as the mechanism of this process is crucial for developing new therapeutic approaches and exploring new therapeutic targets. This review is aimed at exploring the latest findings and roles of Plin5 in lipid metabolism and DC-related pathogenesis, to explore possible clinical intervention approaches.

Perilipin 5 S155 phosphorylation by PKA is required for the control of hepatic lipid metabolism and glycemic control

Perilipin 5 (PLIN5) is a lipid-droplet-associated protein that coordinates intracellular lipolysis in highly oxidative tissues and is thought to regulate lipid metabolism in response to phosphorylation by protein kinase A (PKA). We sought to identify PKA phosphorylation sites in PLIN5 and assess their functional relevance in cultured cells and the livers of mice. We detected phosphorylation on S155 and identified S155 as a functionally important site for lipid metabolism. Expression of phosphorylation-defective PLIN5 S155A in Plin5 null cells resulted in decreased rates of lipolysis and triglyceride-derived fatty acid oxidation. FLIM-FRET analysis of protein-protein interactions showed that PLIN5 S155 phosphorylation regulates PLIN5 interaction with adipose triglyceride lipase at the lipid droplet, but not with α-β hydrolase domain-containing 5. Re-expression of PLIN5 S155A in the liver of Plin5 liver-specific null mice reduced lipolysis compared with wild-type PLIN5 re-expression, but was not associated with other changes in hepatic lipid metabolism. Furthermore, glycemic control was impaired in mice with expression of PLIN5 S155A compared with mice expressing PLIN5. Together, these studies demonstrate that PLIN5 S155 is required for PKA-mediated lipolysis and builds on the body of evidence demonstrating a critical role for PLIN5 in coordinating lipid and glucose metabolism.

Lipid droplet and its implication in cancer progression

Lipid droplets (LDs) are a kind of organelle that is commonly found in eukaryotic cells to store lipids, which encompass a hydrophobic core composed of a single layer of phospholipids and neutral lipids (mainly including triacylglycerol (TAG) and cholesterol ester (CE)) as well as a small amount of proteins. LD accumulation is gradually recognized as a prominent characteristic in a variety of cancers and attracts increasing attention on this field. In this article, we not only summarize the composition, synthesis and decomposition of LD, but also highlight its role in carcinogenesis and malignant development of cancers.

Decoration of myocellular lipid droplets with perilipins as a marker for in vivo lipid droplet dynamics: A super-resolution microscopy study in trained athletes and insulin resistant individuals

In many different cell types neutral lipids can be stored in lipid droplets (LDs). Nowadays, LDs are viewed as dynamic organelles, which store and release fatty acids depending on energy demand (LD dynamics). Proteins like perilipin 2 (PLIN2) and PLIN5 decorate the LD membrane and are determinants of LD lipolysis and fat oxidation, thus affecting LD dynamics. Trained athletes and type 2 diabetes (T2D) patients both have high levels of intramyocellular lipid (IMCL). While IMCL content scales negatively with insulin resistance, athletes are highly insulin sensitive in contrast to T2D patients, the so-called athlete's paradox. Differences in LD dynamics may be an underlying factor explaining the athlete's paradox. We aimed to quantify PLIN2 and PLIN5 content at individual LDs as a reflection of the ability to switch between fatty acid release and storage depending on energy demand. Thus, we developed a novel fluorescent super-resolution microscopy approach and found that PLIN2 protein abundance at the LD surface was higher in T2D patients than in athletes. Localization of adipocyte triglyceride lipase (ATGL) to the LD surface was lower in LDs abundantly decorated with PLIN2. While PLIN5 abundance at the LD surface was similar in athletes and T2D patients, we have observed previously that the number of PLIN5 decorated LDs was higher in athletes, indicating more LDs in close association with mitochondria. Thus, in athletes interaction of LDs with mitochondria was more pronounced and LDs have the protein machinery to be more dynamic, while in T2D patients the LD pool is more inert. This observation contributes to our understanding of the athlete's paradox.

The role of ceramides in metabolic disorders: when size and localization matters

Ceramide accumulation is a hallmark in the manifestation of numerous obesity-related diseases, such as type 2 diabetes mellitus and atherosclerosis. Until the early 2000s, ceramides were viewed as a homogenous class of sphingolipids. However, it has now become clear that ceramides exert fundamentally different effects depending on the specific fatty acyl chain lengths, which are integrated into ceramides by a group of enzymes known as dihydroceramide synthases. In addition, alterations in ceramide synthesis, trafficking and metabolism in specific cellular compartments exert distinct consequences on metabolic homeostasis. Here, we examine the emerging concept of how the intracellular localization of ceramides with distinct acyl chain lengths can regulate glucose metabolism, thus emphasizing their potential as targets in the development of novel and specific therapies for obesity and obesity-associated diseases.

Bone Marrow Adiposity: Basic and Clinical Implications

The presence of adipocytes in mammalian bone marrow (BM) has been recognized histologically for decades, yet, until recently, these cells have received little attention from the research community. Advancements in mouse transgenics and imaging methods, particularly in the last 10 years, have permitted more detailed examinations of marrow adipocytes than ever before and yielded data that show these cells are critical regulators of the BM microenvironment and whole-body metabolism. Indeed, marrow adipocytes are anatomically and functionally separate from brown, beige, and classic white adipocytes. Thus, areas of BM space populated by adipocytes can be considered distinct fat depots and are collectively referred to as marrow adipose tissue (MAT) in this review. In the proceeding text, we focus on the developmental origin and physiologic functions of MAT. We also discuss the signals that cause the accumulation and loss of marrow adipocytes and the ability of these cells to regulate other cell lineages in the BM. Last, we consider roles for MAT in human physiology and disease.

Perilipin 5 alleviates HCV NS5A-induced lipotoxic injuries in liver

BACKGROUND

The homeostasis of lipid droplets (LDs) plays a crucial role in maintaining the physical metabolic processes in cells, and is regulated by many LD-associated proteins, including perilipin 5 (Plin5) in liver. As the putative sites of hepatitis C virus (HCV) virion assembly, LDs are vital to viral infection. In addition, the hepatic LD metabolism can be disturbed by non-structural HCV proteins, such as NS5A, but the details are still inexplicit.

METHODS

HCV NS5A was overexpressed in the livers and hepatocytes of wild-type and Plin5-null mice. BODIPY 493/503 and oil red O staining were used to detect the lipid content in mouse livers and hepatocytes. The levels of lipids, lipid peroxidation and inflammation biomarkers were further determined. Immunofluorescence assay and co-immunoprecipitation assay were performed to investigate the relationship of Plin5 and NS5A.

RESULTS

One week after adenovirus injection, livers expressing NS5A showed more inflammatory cell aggregation and more severe hepatic injuries in Plin5-null mice than in control mice, which was consistent with the increased serum levels of IL-2 and TNF-α (P < 0.05) observed in Plin5-null mice. Moreover, Plin5 deficiency in the liver and hepatocytes aggravated the elevation of MDA and 4-HNE levels induced by NS5A expression (P < 0.01). The triglyceride (TG) content was increased approximately 25% by NS5A expression in the wild-type liver and hepatocytes but was unchanged in the Plin5-null liver and hepatocytes. More importantly, Plin5 deficiency in the liver and hepatocytes exacerbated the elevation of non-esterified fatty acids (NEFAs) stimulated by NS5A expression (P < 0.05 and 0.01 respectively). Using triacsin C to block acyl-CoA biosynthesis, we found that Plin5 deficiency aggravated the NS5A-induced lipolysis of TG. In contrast, Plin5 overexpression in HepG2 cells ameliorated the NS5A-induced lipolysis and lipotoxic injuries. Immunofluorescent staining demonstrated that NS5A expression stimulated the targeting of Plin5 to the surface of the LDs in hepatocytes without altering the protein levels of Plin5. By co-IP, we found that the N-terminal domain (aa 32-128) of Plin5 was pivotal for its binding with NS5A.

CONCLUSIONS

Our data highlight a protective role of Plin5 against hepatic lipotoxic injuries induced by HCV NS5A, which is helpful for understanding the steatosis and injuries in liver during HCV infection.

Super-resolution microscopy localizes perilipin 5 at lipid droplet-mitochondria interaction sites and at lipid droplets juxtaposing to perilipin 2

OBJECTIVE

Intramyocellular lipid droplets (LD) and their coat proteins PLIN2 and PLIN5 are involved in lipolysis, with a putative role for PLIN5 in mitochondrial tethering. Reportedly, these proteins co-localize and cover the surface of the LD. To provide the spatial basis for understanding how these proteins possess their distinct roles, we examined the precise location of PLIN2 and PLIN5 and explored PLIN5 presence at LD-mitochondria contact sites using Stimulated emission depletion (STED) microscopy and correlative light-electron microscopy (CLEM) in human skeletal muscle sections.

METHODS

LDs were stained by MDH together with combinations of mitochondrial proteins and PLINs. Subcellular distribution and co-localization of PLIN proteins and mitochondria was imaged by STED microscopy (Leica TCS SP8) and quantified using Pearson's correlation coefficients and intensity profile plots. CLEM was employed to examine the presence of PLIN5 on mitochondria-LD contact sites.

RESULTS

Both PLIN2 and PLIN5 localized to the LD in a dot-like, juxtaposed fashion rather than colocalizing and covering the entire LD. Both STED and CLEM revealed a high fraction of PLIN5 at the LD-mitochondria interface, but not at mitochondrial cristae, as suggested previously.

CONCLUSION

Using two super-resolution imaging approaches, this is the first study to show that in sections of human skeletal muscle PLIN2 and PLIN5 localize to the LD at distinct sites, with abundance of PLIN5 at LD-mitochondria tethering sites. This novel spatial information uncovers that PLIN proteins do not serve as lipolytic barriers but rather are docking sites for proteins facilitating selective lipase access under a variety of lipolytic conditions.

Widespread expression of perilipin 5 in normal human tissues and in diseases is restricted to distinct lipid droplet subpopulations

Diseases associated with the accumulation of lipid droplets are increasing in western countries. Lipid droplet biogenesis, structure and degradation are regulated by proteins of the perilipin family. Perilipin 5 has been shown to regulate basal lipolysis in oxidative tissues. We examine perilipin 5 in normal human tissues and in diseases using protein biochemical and microscopic techniques. Perilipin 5 was constitutively located at small lipid droplets in skeletal myocytes, cardiomyocytes and brown adipocytes. In addition, perilipin 5 was detected in the epithelia of the gastrointestinal and urogenital tract, especially in hepatocytes, the mitochondria-rich parietal cells of the stomach, tubular kidney cells and ductal cells of the salivary gland and pancreas. Granular cytoplasmic expression, without a lipid droplet-bound localization was detected elsewhere. In cardiomyopathies, in skeletal muscle diseases and during hepatocyte steatogenesis, perilipin 5 was upregulated and localized to larger and more numerous lipid droplets. In steatotic human hepatocytes, perilipin 5 was moderately increased and colocalized with perilipins 1 and 2 but not with perilipin 3 at lipid droplets. In liver diseases implicated in alterations of mitochondria, such as mitochondriopathies, alcoholic liver disease, Wilson's disease and acute liver injury, perilipin 5 was frequently localized to small lipid droplets and less in the cytoplasm. In tumorigenesis, perilipin 5 was especially upregulated in lipo-, leio- and rhabdomyosarcoma and hepatocellular and renal cell carcinoma. In summary, our study provides evidence that perilipin 5 is not restricted to certain cell types but localizes to distinct lipid droplet subpopulations reflecting a possible function in oxidative energy supply in normal tissues and in diseases.

The perilipin family of lipid droplet proteins: Gatekeepers of intracellular lipolysis

Lipid droplets in chordates are decorated by two or more members of the perilipin family of lipid droplet surface proteins. The perilipins sequester lipids by protecting lipid droplets from lipase action. Their relative expression and protective nature is adapted to the balance of lipid storage and utilization in specific cells. Most cells of the body have tiny lipid droplets with perilipins 2 and 3 at the surfaces, whereas specialized fat-storing cells with larger lipid droplets also express perilipins 1, 4, and/or 5. Perilipins 1, 2, and 5 modulate lipolysis by controlling the access of lipases and co-factors of lipases to substrate lipids stored within lipid droplets. Although perilipin 2 is relatively permissive to lipolysis, perilipins 1 and 5 have distinct control mechanisms that are altered by phosphorylation. Here we evaluate recent progress toward understanding functions of the perilipins with a focus on their role in regulating lipolysis and autophagy. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

The Perilipins: Major Cytosolic Lipid Droplet-Associated Proteins and Their Roles in Cellular Lipid Storage, Mobilization, and Systemic Homeostasis

The discovery by Dr. Constantine Londos of perilipin 1, the major scaffold protein at the surface of cytosolic lipid droplets in adipocytes, marked a fundamental conceptual change in the understanding of lipolytic regulation. Focus then shifted from the enzymatic activation of lipases to substrate accessibility, mediated by perilipin-dependent protein sequestration and recruitment. Consequently, the lipid droplet became recognized as a unique, metabolically active cellular organelle and its surface as the active site for novel protein-protein interactions. A new area of investigation emerged, centered on lipid droplets' biology and their role in energy homeostasis. The perilipin family is of ancient origin and has expanded to include five mammalian genes and a growing list of evolutionarily conserved members. Universally, the perilipins modulate cellular lipid storage. This review provides a summary that connects the perilipins to both cellular and whole-body homeostasis.

MiR-145 Regulates Lipogenesis in Goat Mammary Cells Via Targeting INSIG1 and Epigenetic Regulation of Lipid-Related Genes

MicroRNAs (miRNAs) are noncoding RNA molecules that regulate gene expression at the post-transcriptional level to cause translational repression or degradation of targets. The profiles of miRNAs across stages of lactation in small ruminant species such as dairy goats is unknown. A small RNA library was constructed using tissue samples from mammary gland of Saanen dairy goats harvested at mid-lactation followed by sequencing via Solexa technology. A total of 796 conserved miRNAs, 263 new miRNAs, and 821 pre-miRNAs were uncovered. After comparative analyses of our sequence data with published mammary gland transcriptome data across different stages of lactation, a total of 37 miRNAs (including miR-145) had significant differences in expression over the lactation cycle. Further studies revealed that miR-145 regulates metabolism of fatty acids in goat mammary gland epithelial cells (GMEC). Compared with nonlactating mammary tissue, lactating mammary gland had a marked increase in expression of miR-145. Overexpression of miR-145 increased transcription of genes associated with milk fat synthesis resulting in greater fat droplet formation, triacylglycerol accumulation, and proportion of unsaturated fatty acids. In contrast, silencing of miR-145 impaired fatty acid synthesis. Inhibition of miR-145 increased methylation levels of fatty acid synthase (FASN), stearoyl-CoA desaturase 1 (SCD1), peroxisome proliferator-activated receptor gamma (PPARG), and sterol regulatory element binding transcription factor 1 (SREBF1). Luciferase reporter assays confirmed that insulin induced gene 1 (INSIG1) is a direct target of miR-145. These findings underscore the need for further studies to evaluate the potential for targeting miR-145 for improving beneficial milk components in ruminant milk. J. Cell. Physiol. 232: 1030-1040, 2017. © 2016 Wiley Periodicals, Inc.

Nuclear Perilipin 5 integrates lipid droplet lipolysis with PGC-1α/SIRT1-dependent transcriptional regulation of mitochondrial function

Dysfunctional cellular lipid metabolism contributes to common chronic human diseases, including type 2 diabetes, obesity, fatty liver disease and diabetic cardiomyopathy. How cells balance lipid storage and mitochondrial oxidative capacity is poorly understood. Here we identify the lipid droplet protein Perilipin 5 as a catecholamine-triggered interaction partner of PGC-1α. We report that during catecholamine-stimulated lipolysis, Perilipin 5 is phosphorylated by protein kinase A and forms transcriptional complexes with PGC-1α and SIRT1 in the nucleus. Perilipin 5 promotes PGC-1α co-activator function by disinhibiting SIRT1 deacetylase activity. We show by gain-and-loss of function studies in cells that nuclear Perilipin 5 promotes transcription of genes that mediate mitochondrial biogenesis and oxidative function. We propose that Perilipin 5 is an important molecular link that couples the coordinated catecholamine activation of the PKA pathway and of lipid droplet lipolysis with transcriptional regulation to promote efficient fatty acid catabolism and prevent mitochondrial dysfunction.

MicroRNA-26a/b and their host genes synergistically regulate triacylglycerol synthesis by targeting the INSIG1 gene

The microRNA-26 (miR-26) family is known to control adipogenesis in non-ruminants. The genomic loci of miR-26a and miR-26b have been localized in the introns of genes encoding for the proteins of the C-terminal domain RNA polymerase II polypeptide A small phosphatase (CTDSP) family. Insulin-induced gene 1 (INSIG1) encodes a protein with a key role in the regulation of lipogenesis in rodent liver. In the present study, we investigated the synergistic function of the miR-26 family and their host genes in goat mammary epithelial cells (GMEC). Downregulation of miR-26a/b and their host genes in GMEC decreased the expression of genes relate to fatty acid synthesis (PPARG, LXRA, SREBF1, FASN, ACACA, GPAM, LPIN1, DGAT1 and SCD1), triacylglycerol accumulation and unsaturated fatty acid synthesis. Luciferase reporter assays confirmed INSIG1 as a direct target of miR-26a/b. Furthermore, inhibition of the CTDSP family also downregulated the expression of INSIG1. Taken together, our findings highlight a functional association of miR-26a/b, their host genes and INSIG1, and provide new insights into the regulatory network controlling milk fat synthesis in GMEC. The data indicate that targeting this network via nutrition might be important for regulating milk fat synthesis in ruminants.

Perilipin 5 mediated lipid droplet remodelling revealed by coherent Raman imaging

Accumulation of fat in muscle tissue as intramyocellular lipids (IMCLs) is closely related to the development of insulin resistance and subsequent type 2 diabetes. Most IMCLs organize into lipid droplets (LDs), the fates of which are regulated by lipid droplet coat proteins. Perilipin 5 (PLIN5) is an LD coating protein, which is strongly linked to lipid storage in muscle tissue. Here we employ a tandem in vitro/ex vivo approach and use chemical imaging by label-free, hyperspectral coherent Raman microscopy to quantify compositional changes in individual LDs upon PLIN5 overexpression. Our results directly show that PLIN5 overexpression in muscle alters individual LD composition and physiology, resulting in larger LDs with higher esterified acyl chain concentration, increased methylene content, and more saturated lipid species. These results suggest that lipotoxic protection afforded by natural PLIN5 upregulation in muscle involves molecular changes in lipid composition within LDs.

MicroRNA-24 can control triacylglycerol synthesis in goat mammary epithelial cells by targeting the fatty acid synthase gene

In nonruminants it has been demonstrated that microRNA-24 (miR-24) is involved in preadipocyte differentiation, hepatic lipid, and plasma triacylglycerol synthesis. However, its role in ruminant mammary gland remains unclear. In this study we measured miR-24 expression in goat mammary gland tissue at 4 different stages of lactation and observed that it had highest expression at peak lactation when compared with the dry period. Overexpression or downregulation of miR-24 in goat mammary epithelial cells (GMEC) strongly affected fatty acid profiles; in particular, miR-24 enhanced unsaturated fatty acid concentration. Additional effects of miR-24 included changes in triacylglycerol content and the expression of fatty acid synthase, sterol regulatory element binding transcription protein 1, stearoyl-CoA desaturase, glycerol-3-phosphate acyltransferase mitochondrial, and acetyl-CoA carboxylase. Luciferase reporter assay confirmed that fatty acid synthase is a target of miR-24. Taken together, these results not only highlight the physiological importance of miR-24 in fatty acid metabolism in GMEC, but also laid the foundation for further research on regulatory mechanisms among miR-24 and other microRNA expressed in GMEC.

Unraveling the roles of PLIN5: linking cell biology to physiology

The discovery of perilipin (PLIN) 1 provided a major conceptual shift in the understanding of adipose tissue lipolysis and generated intense interest in lipid droplet biology research. The subsequent discovery of other PLIN proteins revealed unique tissue distribution profiles, subcellular locations, and lipid-binding properties and divergent cellular functions. PLIN5 is highly expressed in oxidative tissues such as skeletal muscle, liver, and heart and is central to lipid homeostasis in these tissues. Studies in cell systems have ascribed several metabolic roles to PLIN5 and demonstrated interactions with other proteins that are requisite for these functions. We examine recent in vivo studies and ask whether the evidence from the cell biology approaches is consistent with the physiological roles of PLIN5.

Higher PLIN5 but not PLIN3 content in isolated skeletal muscle mitochondria following acute in vivo contraction in rat hindlimb

Contraction-mediated lipolysis increases the association of lipid droplets and mitochondria, indicating an important role in the passage of fatty acids from lipid droplets to mitochondria in skeletal muscle. PLIN3 and PLIN5 are of particular interest to the lipid droplet-mitochondria interaction because PLIN3 is able to move about within cells and PLIN5 associates with skeletal muscle mitochondria. This study primarily investigated: 1) if PLIN3 is detected in skeletal muscle mitochondrial fraction; and 2) if mitochondrial protein content of PLIN3 and/or PLIN5 changes following stimulated contraction. A secondary aim was to determine if PLIN3 and PLIN5 associate and whether this changes following contraction. Male Long Evans rats (n = 21; age, 52 days; weight = 317 ± 6 g) underwent 30 min of hindlimb stimulation (10 msec impulses, 100 Hz/3 sec at 10-20 V; train duration 100 msec). Contraction induced a ~50% reduction in intramuscular lipid content measured by oil red-O staining of red gastrocnemius muscle. Mitochondria were isolated from red gastrocnemius muscle by differential centrifugation and proteins were detected by western blotting. Mitochondrial PLIN5 content was ~1.6-fold higher following 30 min of contraction and PLIN3 content was detected in the mitochondrial fraction, and unchanged following contraction. An association between PLIN3 and PLIN5 was observed and remained unaltered following contraction. PLIN5 may play a role in mitochondria during lipolysis, which is consistent with a role in facilitating/regulating mitochondrial fatty acid oxidation. PLIN3 and PLIN5 may be working together on the lipid droplet and mitochondria during contraction-induced lipolysis.

ERα regulates lipid metabolism in bone through ATGL and perilipin

A decrease in bone mineral density during menopause is accompanied by an increase in adipocytes in the bone marrow space. Ovariectomy also leads to accumulation of fat in the bone marrow. Herein we show increased lipid accumulation in bone marrow from estrogen receptor alpha (ERα) knockout (ERαKO) mice compared to wild-type (WT) mice or estrogen receptor beta (ERβ) knockout (ERβKO) mice. Similarly, bone marrow cells from ERαKO mice differentiated to adipocytes in culture also have increased lipid accumulation compared to cells from WT mice or ERβKO mice. Analysis of individual adipocytes shows that WT mice have fewer, but larger, lipid droplets per cell than adipocytes from ERαKO or ERβKO animals. Furthermore, higher levels of adipose triglyceride lipase (ATGL) protein in WT adipocytes correlate with increased lipolysis and fewer lipid droplets per cell and treatment with 17β-estradiol (E2) potentiates this response. In contrast, cells from ERαKO mice display higher perilipin protein levels, promoting lipogenesis. Together these results demonstrate that E2 signals via ERα to regulate lipid droplet size and total lipid accumulation in the bone marrow space in vivo.

PLIN5 deletion remodels intracellular lipid composition and causes insulin resistance in muscle

Defective control of lipid metabolism leading to lipotoxicity causes insulin resistance in skeletal muscle, a major factor leading to diabetes. Here, we demonstrate that perilipin (PLIN) 5 is required to couple intramyocellular triacylglycerol lipolysis with the metabolic demand for fatty acids. PLIN5 ablation depleted triacylglycerol stores but increased sphingolipids including ceramide, hydroxylceramides and sphingomyelin. We generated perilipin 5 (Plin5)(-/-) mice to determine the functional significance of PLIN5 in metabolic control and insulin action. Loss of PLIN5 had no effect on body weight, feeding or adiposity but increased whole-body carbohydrate oxidation. Plin5 (-/-) mice developed skeletal muscle insulin resistance, which was associated with ceramide accumulation. Liver insulin sensitivity was improved in Plin5 (-/-) mice, indicating tissue-specific effects of PLIN5 on insulin action. We conclude that PLIN5 plays a critical role in coordinating skeletal muscle triacylglycerol metabolism, which impacts sphingolipid metabolism, and is requisite for the maintenance of skeletal muscle insulin action.

Cellular localization and associations of the major lipolytic proteins in human skeletal muscle at rest and during exercise

Lipolysis involves the sequential breakdown of fatty acids from triacylglycerol and is increased during energy stress such as exercise. Adipose triglyceride lipase (ATGL) is a key regulator of skeletal muscle lipolysis and perilipin (PLIN) 5 is postulated to be an important regulator of ATGL action of muscle lipolysis. Hence, we hypothesized that non-genomic regulation such as cellular localization and the interaction of these key proteins modulate muscle lipolysis during exercise. PLIN5, ATGL and CGI-58 were highly (>60%) colocated with Oil Red O (ORO) stained lipid droplets. PLIN5 was significantly colocated with ATGL, mitochondria and CGI-58, indicating a close association between the key lipolytic effectors in resting skeletal muscle. The colocation of the lipolytic proteins, their independent association with ORO and the PLIN5/ORO colocation were not altered after 60 min of moderate intensity exercise. Further experiments in cultured human myocytes showed that PLIN5 colocation with ORO or mitochondria is unaffected by pharmacological activation of lipolytic pathways. Together, these data suggest that the major lipolytic proteins are highly expressed at the lipid droplet and colocate in resting skeletal muscle, that their localization and interactions appear to remain unchanged during prolonged exercise, and, accordingly, that other post-translational mechanisms are likely regulators of skeletal muscle lipolysis.

Perilipin 5, a lipid droplet protein adapted to mitochondrial energy utilization

PURPOSE OF REVIEW

We summarize recent mechanistic and physiological studies related to the role of perilipin 5 (Plin5) in regulating lipid droplet accumulation and protection to fatty acids in tissues with high lipid oxidative metabolism.

RECENT FINDINGS

Plin5 is a lipid droplet targeting protein that promotes association of lipid droplets with mitochondria and is most highly expressed in oxidative tissues, including cardiac and skeletal muscle. Recent in-vivo and in-vitro data indicate an important role for Plin5 in the regulation of cardiac lipid storage and function. Targeted overexpression of Plin5 in heart causes steatosis, mild mitochondria dysfunction, and hypertrophy in cardiac tissue, but without affecting cardiac function. In contrast, whole body ablation of Plin5 (Plin5  mice) reduces cardiac lipid droplet formation, increases cardiac fatty acid oxidation, and promotes cardiac dysfunction; cardiac defects can be prevented with antioxidative therapy. These data suggest a cytoprotective role for Plin5 to promote lipid storage but to limit fatty acid toxicity, parameters critical for tissues with high lipid oxidative metabolism.

SUMMARY

In-vivo and in-vitro data suggest that Plin5 is part of a cell-adaptive response to high lipid oxidative metabolism to protect lipid droplet storage against neutral lipases and, so, limit fatty acid accumulation. Although the specific mechanisms that underlie Plin5 lipid droplet storage protection in oxidative tissues remain to be fully elucidated, Plin5 provides a basis for the novel cytoprotective nature of lipid droplets.

Lipid droplet metabolism

PURPOSE OF REVIEW

With the realization that lipid droplets are not merely inert fat storage organelles, but highly dynamic and actively involved in cellular lipid homeostasis, there has been an increased interest in lipid droplet biology. Recent studies have begun to unravel the roles that lipid dropletss play in cellular physiology and provide insights into the mechanisms by which lipid droplets contribute to cellular homeostasis. This review provides a summary of these recent publications on lipid droplet metabolism.

RECENT FINDINGS

Perilipins have different preferences for associating with triacylglycerol (TAG) or cholesteryl esters, different tissue distributions, and each contributes to lipid metabolism in its unique way. Cell death-inducing DFF45-like effector proteins are not only involved in lipid droplet expansion, but also in the cellular response to stress and lipid secretion. Lipid droplets undergo an active cycle of lipolysis and re-esterification to form microlipid droplets. TAG synthesis for lipid droplet formation and expansion occurs in the endoplasmic reticulum and on lipid droplets, and TAG transfers between lipid droplets during lipid droplet fusion. Lipid droplets interact with the endoplasmic reticulum and mitochondria to facilitate lipid transfer, lipid droplet expansion, and metabolism.

SUMMARY

Lipid droplets are dynamically active, responding to changes in cellular physiology, as well as interacting with cytosolic proteins and other organelles to control lipid homeostasis.

Perilipins: lipid droplet coat proteins adapted for tissue-specific energy storage and utilization, and lipid cytoprotection

Cytosolic lipid storage droplets are primary functional organelles that regulate cellular lipid metabolism and homeostasis. Paradoxically, excess lipid stores are linked to both adaptive (fasting and chronic exercise) and mal-adaptive (obesity and related health complications) conditions. Thus, collective metabolic and physiological processes must balance lipid storage and utilization with prevention of lipocytotoxicity and compounding tissue dysfunctions, urging the need to further define the connection of mammalian lipid droplet function and lipid homeostasis. The perilipins are a multi-protein family that targets lipid droplet surfaces and regulates lipid storage and hydrolysis. Study of perilipin functions has provided insight into the physiological roles of cytosolic lipid droplets and their relationship with obesity-related pathologies. Here, we review the current knowledge of the multiple perilipin proteins in regulating tissue-specific lipid droplets and associations with tissue and systemic energetics.

Perilipin 1 moves between the fat droplet and the endoplasmic reticulum

Perilipin 1, unlike the other perilipins, is thought to be restricted to the fat droplet. We reassessed its cellular distribution using the fat droplet marker CGI-58 in OP9 and 3T3-L1 adipocyte lines and in brown adipose tissue (BAT). As expected, we found perilipin 1 in the fat droplet-enriched floating fraction from centrifuged adipocyte or BAT homogenates. However, about half of perilipin 1 was suspended in the cytosol/infranate or pelleted with cellular membranes. In these fractionations, most of the fat droplet-associated protein CGI-58 was in the floating fraction. In BAT and OP9 adipocytes about a third of perilipin 1 pellets, compared with a much smaller fraction of CGI-58. Co-imaging perilipin 1 and smooth endoplasmic reticulum (ER) markers reveals both ER and fat droplet associated perilipin 1 in OP9 adipocytes. Consistent with these observations, perilipin 1 overexpressed in COS7 cells mostly fractionates with cellular membranes and imaging shows it on the ER. In 3T3-L1 adipocytes almost half of perilipin 1 floats, half is suspended as infranate and small amounts pellet. Finally, driving rapid fat droplet synthesis in OP9 adipocytes increases the intensity of perilipin 1 on fat droplets, while decreasing non-fat droplet immunolabeling. Confirming the morphological findings, fractionation shows perilipin 1 moving from the pelleted to the floated fractions. In conclusion, this study documents an expanded intracellular distribution for perilipin 1 and its movement from ER to fat droplet during lipid synthesis.

Cardiac-specific overexpression of perilipin 5 provokes severe cardiac steatosis via the formation of a lipolytic barrier

Cardiac triacylglycerol (TG) catabolism critically depends on the TG hydrolytic activity of adipose triglyceride lipase (ATGL). Perilipin 5 (Plin5) is expressed in cardiac muscle (CM) and has been shown to interact with ATGL and its coactivator comparative gene identification-58 (CGI-58). Furthermore, ectopic Plin5 expression increases cellular TG content and Plin5-deficient mice exhibit reduced cardiac TG levels. In this study we show that mice with cardiac muscle-specific overexpression of perilipin 5 (CM-Plin5) massively accumulate TG in CM, which is accompanied by moderately reduced fatty acid (FA) oxidizing gene expression levels. Cardiac lipid droplet (LD) preparations from CM of CM-Plin5 mice showed reduced ATGL- and hormone-sensitive lipase-mediated TG mobilization implying that Plin5 overexpression restricts cardiac lipolysis via the formation of a lipolytic barrier. To test this hypothesis, we analyzed TG hydrolytic activities in preparations of Plin5-, ATGL-, and CGI-58-transfected cells. In vitro ATGL-mediated TG hydrolysis of an artificial micellar TG substrate was not inhibited by the presence of Plin5, whereas Plin5-coated LDs were resistant toward ATGL-mediated TG catabolism. These findings strongly suggest that Plin5 functions as a lipolytic barrier to protect the cardiac TG pool from uncontrolled TG mobilization and the excessive release of free FAs.

Expression of oleosin and perilipins in yeast promote formation of lipid droplets from the endoplasmatic reticulum

Most cells store neutral lipids in a dedicated compartment, the lipid droplet (LD). These LDs are structurally and functionally conserved across species. In higher eukaryotes, LDs are covered by abundant scaffolding proteins, such as the oleosins in plants and perilipins (PLINs) in animal cells. S. cerevisiae, however, has no homologues of these scaffolding proteins. To analyze a possible function of these proteins in the biogenesis of LDs, oleosin and perilipin family members (PLIN1, ADRP/PLIN2, and TIP47/PLIN3) were expressed in yeast cells and their targeting to LDs, membrane association and function in neutral lipid homeostasis and LD biogenesis were analyzed. When expressed in wild-type cells, these proteins were properly targeted to LDs. However, when expressed in cells lacking LDs, oleosin was localized to the ER bilayer and was rapidly degraded. PLINs, on the other hand, did not localize to the ER membrane in the absence of LDs and lost their membrane association. Photobleaching experiments revealed that PLIN2 and PLIN3 rapidly exchanged their LD association but PLINs did not move over the LD surface as quickly as did an integral membrane protein, such as oleosin. Interestingly, expression of these scaffolding LD proteins in mutant cells containing elevated levels of neutral lipids within the ER bilayer resulted in the formation of LDs. These results suggest that these LD scaffolding proteins promote the sequestration of neutral lipids from the ER bilayer and thereby induce LD formation. Consistent with this proposition, addition of a cell permeable diacylglycerol (DAG) was sufficient to promote LD formation in cells expressing the LD scaffolding proteins but lacking the capacity to synthesize storage lipids.

Perilipin family (PLIN) proteins in human skeletal muscle: the effect of sex, obesity, and endurance training

Proteins that coat the lipid droplets (also known as PAT proteins or perilipin (PLIN) family proteins) have diverse functions that are not well elucidated in many tissues. In skeletal muscle, there is even less known about the functions or characteristics of these proteins or how they might change in response to perturbations that alter both intramyocellular lipid (IMCL) content and fat utilization and oxidation. Therefore, the purpose of this study was to examine the human muscle content and gene expression of the four skeletal muscle PLIN proteins in both lean and obese men and women and how this was changed following a 12-week endurance training protocol. PLIN2–PLIN5 proteins were all more abundant in women than in men (p = 0.037 and p < 0.0001, respectively), consistent with higher IMCL content observed in female skeletal muscle. PLIN5 (previously known as OXPAT) is of particular interest because it has previously been associated primarily with oxidative tissues that rely heavily on fat oxidation for energy production. Although PLIN5 was not different between lean and obese subjects, it was the only PLIN protein to increase in response to endurance training in both sexes. PLIN5 correlated with IMCL volume (p < 0.0001), but in general, the other PLIN proteins did not correlate well with IMCL volume, suggesting that the relationship between lipid accumulation and PLIN family protein content is not a simple one. Although more work is necessary, it is clear that PLIN5 likely plays an important role in IMCL accumulation and oxidation, both of which increase with endurance training in human skeletal muscle.

Perilipin 5, a lipid droplet-binding protein, protects heart from oxidative burden by sequestering fatty acid from excessive oxidation

Lipid droplets (LDs) are ubiquitous organelles storing neutral lipids, including triacylglycerol (TAG) and cholesterol ester. The properties of LDs vary greatly among tissues, and LD-binding proteins, the perilipin family in particular, play critical roles in determining such diversity. Overaccumulation of TAG in LDs of non-adipose tissues may cause lipotoxicity, leading to diseases such as diabetes and cardiomyopathy. However, the physiological significance of non-adipose LDs in a normal state is poorly understood. To address this issue, we generated and characterized mice deficient in perilipin 5 (Plin5), a member of the perilipin family particularly abundant in the heart. The mutant mice lacked detectable LDs, containing significantly less TAG in the heart. Particulate structures containing another LD-binding protein, Plin2, but negative for lipid staining, remained in mutant mice hearts. LDs were recovered by perfusing the heart with an inhibitor of lipase. Cultured cardiomyocytes from Plin5-null mice more actively oxidized fatty acid than those of wild-type mice. Production of reactive oxygen species was increased in the mutant mice hearts, leading to a greater decline in heart function with age. This was, however, reduced by the administration of N-acetylcysteine, a precursor of an antioxidant, glutathione. Thus, we conclude that Plin5 is essential for maintaining LDs at detectable sizes in the heart, by antagonizing lipase(s). LDs in turn prevent excess reactive oxygen species production by sequestering fatty acid from oxidation and hence suppress oxidative burden to the heart.